Interest has increased in transmitting acoustic signals to and from locations in an oil well environment. The basic operating principal in acoustic signal transmission in a tubular media is to impart propagating stress waves into a pipe or tubing string which travel within the pipe to a distant location where transducers detect the signal which is then interpreted by the receiving equipment. In this way, data and signals can be transmitted via mechanical tubular transmission channels such as pipe or tubing.
There are many practical problems associated with using this scheme. When tubing, drill pipe or casing is used as an acoustic transmission channel, there is often significant signal distortion due to reflective interfaces in the channel such as tool joints, collars or other upsets. Additionally, there can be significant attenuation and interference associated with the fluid system within the wellbore and echos of the acoustic signals themselves within the wellbore. Unwanted interfering signals caused by external disturbance sources may also be present in the acoustic channel. These factors significantly reduce the conditions under which acoustic data transmission may be effectively utilized. Acoustic data transmission may be limited by the distance of the transmission, the number and type of upsets in a drill string.
Efforts to effectively transmit data acoustically have often centered on careful control of the frequency and bandwidth of the transmission, the timing of the transmission and the duration of the transmission. U.S. Pat. No. 3,252,225 issued to Hixon and U.S. Pat. No. 4,314,365 issued to Petersen teach selection of transmission wave length based upon pipe characteristics such as the length of pipe sections and the overall length of the drill string. U.S. Pat. No. 4,390,975 issued to Shawhan suggests delaying successive acoustic data transmissions to allow reflections of earlier transmissions to dissipate. Similarly, U.S. Pat. No. 5,050,132 issued to Duckworth discloses transmissions of acoustic data signals only during preselected short time intervals to avoid data distortion. U.S. Pat. No. 5,124,953 issued to Grosso discloses selecting a passband frequency for acoustic data transmission that best correlates a measured and a modeled Apower spectral density of the acoustic transmission. U.S. Pat. No. 5,148,408 issued to Matthews similarly suggests the testing and finding of an optimum frequency for acoustic data transmission which results in the most efficient reception of the acoustic data under the circumstances then present in the well. The Matthews patent suggested period testing of data transmission through the drill string during drilling operations, finding an optimum frequency for transmission based upon drill string conditions at the time of testing, and changing the acoustic data transmission frequency as needed. U.S. Pat. No. 4,562,559 issued to Sharp et al, proposes a phase-shifted transmission wave having a broader frequency spectrum to bridge gaps in the passbands. U.S. Pat. No. 5,128,901 issued to Drumheller proposes transmission of acoustic data conditioned to counteract interference caused by the drill string. Prior to transmission, each signal frequency is multiplied by a factor designed to enhance data transmission.
In many communications systems it is possible to model the communication channel before the system is placed in service, then to design an acoustic transmitter to compensate for the channel distortion. Unfortunately, in an oil well the acoustic transmission environment changes continuously, so it is impossible to design a static acoustic transmitter, which is tailored to the oil well environment. Further complicating acoustic equalization is the complex acoustic environment in an oil well which often contains non-linearities which cannot be effectively modeled using linear filtering techniques.
From the foregoing, it is apparent that a need exists for improved methods of acoustic data transmission and, in particular, a need exists for utilizing such improved methods of acoustic data transmission in oil well environments. Furthermore, it would be desirable to provide such methods, which compensate for changes in the environments in which the acoustic data transmission occurs.